The development of a power electronic device is orientated to miniaturization, lightweight, high efficiency, and low cost. In the conventional power supply systems, a power semiconductor device is in a hard switching operation state, and there is a larger overlapping area between voltage and current, causing problems such as large switch loss, electromagnetic interference noise, ringing and the like, thus resulting in high cost, huge volume and low efficiency of the systems. If high performance is desired to be available, high cost is needed. With fiercer competition in power electronics market, the semiconductor industry has developed rapidly, but high-performance devices have not yet been used in large scale, so the study on soft switching circuit topology is still a selection for most of power supply manufactures to improve product competitiveness.
FIG. 1 shows a conventional circuit topology of the three-phase soft switching NPC1-type inverter in the prior art, which is composed of three NPC1 bridge arms, including an S-phase bridge arm, a R-phase bridge arm and a T-phase bridge arm, wherein the R-phase bridge arm comprises a main switch R.up1, a main switch R.up2, a main switch R.down2, and a main switch R.down1; the S-phase bridge arm comprises a main switch S.up1, a main switch S.up2, a main switch S.down2, and a main switch S.down1; and the T-phase bridge arm comprises a main switch T.up1, a main switch T.up2, a main switch T.down2, and a main switch T.down1. Each bridge arm is composed of upper and lower bridge arms (for example, an upper bridge arm of the R-phase bridge arm comprises the main switch R.up1 and the main switch R.up2, and a lower bridge arm of the R-phase bridge arm comprises the main switch R.down1 and the main switch R.down2) for controlling positive and negative half cycles respectively, the four switches may be semiconductor devices such as an IGBT, an MOSFET, a GTO or an SCR or the like, and diodes R.d1, R.d2 are used for performing the functions such as clamping, Buck output freewheeling, Boost output energy storage and the like for output point voltage. Diodes connected in parallel with main switches (power switches) are body diodes or external diodes. Capacitances in parallel with the main switches are junction capacitances or are externally connected in parallel. The magnitude of output filtering capacitances C.R, C.S and C.T and inductors L.R, L.S and L.T is determined by factors such as system power and switching frequency and the like. An auxiliary switch circuit comprises auxiliary switches Up.Aux1, Down.Aux1, coupling inductors Tx.up.Aux, Tx.Down.Aux and the like, and are provided to realize zero voltage switching on (ZVS) of main power switch devices and soft recovery of diodes. Junction capacitances connected in parallel with main power devices (i.e., main switches) can soften a turn-off process, and act as key elements of resonance in a turn-on process. A plurality of bridge arms can realize zero voltage switching on (ZVS) of three-phase main power switch devices via diode logic wired AND. An auxiliary switch drive circuit varies depending on control method and logic, and various auxiliary switch drive circuit differ for the system to realize soft switching principle and efficiency improvement. A difference between BUS+ and BUS− is bus voltage. BUS.C1 and BUS.C2 are positive and negative voltage-stabilized bus capacitances, respectively, and may be regarded as constant voltage sources during analysis.
By the auxiliary switch circuit, activated current enters the auxiliary switch circuit, such that a switching process of freewheeling current of the main switch circuit towards energy storage current is softened, so as to reduce an overlapping area between voltage and current to the largest extent.
FIG. 2 is an operation waveform diagram of a single-phase main switch circuit of the three-phase soft switching NPC1-type inverter in the prior art, and FIG. 3 is an operation waveform diagram of a single-phase auxiliary switch circuit of the three-phase soft switching NPC1-type inverter in the prior art.
Referring to FIG. 1 to FIG. 3, supposing that the R-phase output voltage is in a positive half cycle and that PF=1, the positive bridge arm operates in a Buck operation mode, the main switch R.up1 is a high-frequency switch, and the main switch R.down1 is a low-frequency switch and is conductive all the time. Supposing that in an initial state the freewheeling diode R.d1 is conductive to release magnetism, and the auxiliary switch Up.Aux1 and the main switch R.up1 are turned off, current flows out of the freewheeling diode, the main switch R.down1 and the inductor L.R.
Model1 (t1˜t2): the auxiliary switch Up.Aux1 is turned on; since the coupling inductor Tx.up.Aux limits a rising rate di/dt of current in the auxiliary switch circuit, the turn-on process of the auxiliary switch Up.Aux1 can be softened, and meanwhile the coupling inductor Tx.up.Aux also reduces a declining rate di/dt of current of the freewheeling diode R.d1, suppressing a reverse recovery current of the freewheeling diode R.d1 effectively, thus realizing soft recovery of the freewheeling diode R.d1. Due to the current sharing function of the coupling inductor Tx.up.Aux, a current stress of the auxiliary switch Up.Aux1 will be assigned to each auxiliary switch equally, so as to reduce an auxiliary current stress.
Model2 (t2˜t3): after the auxiliary switch Up.Aux1 is turned on for a period of time, current is completely transferred to the auxiliary switch Up.Aux1, accompanied by LC resonance, so as to realize charging of the capacitor connected in parallel with the main switch R.up1 with the upper portion being positive and the lower portion being negative, wherein when capacitor voltage is greater than conductive voltage of the body diode of the main switch R.up1, forward conduction of the body diode of the main switch R.up1 is realized, such that the LC resonance terminates, and at this time the main switch R.up1 is controlled to be turned on, and meanwhile zero voltage switching on of the main switch R.up1 can be realized, thereby starting a Buck magnetizing mode of the main switch.
Model3 (t3˜t4): after the zero voltage switching on of the main switch R.up1, current is transferred from the auxiliary switch circuit to the main switch circuit; upon complete transfer of the current, the auxiliary switch Up.Aux1 is turned off; since the main switch R.up1 performs a function of turning on and in turn clamping, approximate zero-voltage zero-current turn-off of the auxiliary switch can be realized, and in view of the unidirectional current flow characteristic of diode D.up.Aux1, natural inversion of the auxiliary diode can be realized after the LC resonance reaches zero.
Model4 (t4˜t5): when it is required to turn off the main switch R.up1, due to the characteristic that capacitance voltage cannot change abruptly and the characteristic that the LC resonance has small damping, LC resonates with high frequencies, also realizing reverse charging of the junction capacitance of the main switch R.up1, thus making it possible to soften the turn-off process of the main switch R.up1. At this time, the auxiliary switch circuit and the main freewheeling diode simultaneously undertake the functions of freewheeling and magnetism releasing.
Model5 (t5˜t6): after the main switch R.up1 is turned off, also due to the unidirectional current flow characteristic of diode D.up.Aux1, the LC resonance ends, and the main freewheeling diode undertakes all the freewheeling and magnetizing processes.
Model6 (t0˜t6): one switching cycle ends, and a next switching cycle starts. The process is as described above.
The operating principle of the respective devices in a negative half cycle of the output voltage is the same as the operating principle of the respective devices in the positive half cycle thereof. The operating principles of the remaining two phases are the same as the operating principle of the R-phase, and will not be described repeatedly.
In FIG. 2 and FIG. 3, R.up1.ge, Up.Aux1.ge, P.sw.up1, R.down1.i, R.d1.V, R.d1.i, R.up1.i, R.up1.Vce, Up.Aux1.Vce, Up.Aux1.i, Tx.up.Aux.s.i, Tx.up.Aux.p.i, and D.up.Aux1.i respectively represent: control level of the main switch R.up1, control level of the auxiliary switch Up.Aux1, loss of the main switch R.up1, current of the main switch R.down1, voltage of the freewheeling diode R.d1, current of the freewheeling diode R.d1, current of the main switch R.up1, ce voltage drop of the main switch R.up1, ce voltage drop of the auxiliary switch Up.Aux1, current of the auxiliary switch Up.Aux1, current in primary side S of the coupling inductor Tx.up.Aux, current in secondary side p of the coupling inductor Tx.up.Aux, and current of the diode D.up.Aux1. Compared with a three-phase hard switching NPC1-type inverter, turn-on and turn-off loss of the main switch R.up1 of the three-phase soft switching NPC1-type inverter is reduced. As can be seen from FIG. 3, the primary side current Tx.up.Aux.s.i and the secondary side current Tx.up.Aux.p.i of the coupling inductor are equal, and are ½ of D.up.Aux1.i. 
The following conclusion can be reached from the above analysis: the soft switching topology of the zero voltage soft switching converter can realize zero voltage switching of the main power switches and soft recovery of the diodes, realize soft turn-off of the main power devices through capacitances in parallel connection, thereby improving system efficiency, and can realize soft switching on of the main switches and zero-current zero-voltage turn-off of the auxiliary switches, and natural inversion of the auxiliary diodes.
In an AC/DC or DC/AC power source, within one switching cycle, it is typical to combine basic voltage vectors such that an average value thereof is equal to a given voltage vector; at a certain time the voltage vectors are rotated and combined such that an average value thereof is equal to a given voltage vector; at a certain time the voltage vectors are rotated into a certain area, which can be obtained by different combinations in terms of time of two neighboring non-zero vector and zero vector constituting this area, action times of the two vectors are applied multiple times within one use cycle, thereby controlling action times of the respective voltage vectors, such that voltage space vectors are rotated approximately in a circular trajectory to generate corresponding PWM waveforms. The PWM waveforms drive actions of the switches, such that given waveforms desired can be obtained through LC filtering.
In the conventional PWM control methods, carriers have a plurality of control freedom degrees such as frequency, phase, amplitude, offset amount and the like. Modulation waves also at least have a plurality of control freedom degrees such as frequency, amplitude, zero-sequence component, shape and the like, and different combinations of these control freedom degrees can generate a large number of PWM control methods.
Although the above existing three-phase soft switching NPC1-type inverter has many advantages, overall machine loss is still relatively large, making it difficult to further improve system efficiency.